Rechargeable batteries use polymer binders to hold together the active particulate material and adhere this particulate material to the current collector in the fabrication of electrodes. The binder is generally comprised of one or more polymers. The binders commonly used in commercial li-ion batteries are polyvinyledene fluoride (PVDF), ethylene-propylene and a diene (EPDM). These polymers are generally insoluble in water and, thus are dissolved in an organic solvent such as N-methyl pyrrolidone (NMP). The organic solvent additionally serves as a dispersion medium for the active materials. Some disadvantages of using organic solvents are that they have relatively high cost, can possess negative environmental impacts, and pose disposal issues. Further, PVDF is highly unstable and tends to break down at high temperatures.
Known water soluble binders, such as carboxy methyl cellulose (CMC), require a thickening agent to control the viscosity of the binder. Further, they exhibit only marginal adhesion capability. Polytetrafluoroethylene (PTFE) and styrene butadiene rubber (SBR) based water soluble binders also exhibit poor adhesion and do not exhibit good cycle life. Further, SBR binders exhibit high expandability and undesirable agglomeration characteristics resulting in poor dispersion, poor performance, and high electrode resistance.
Accordingly, there is a need for a water soluble binder in rechargeable lithium batteries. This water soluble binder should exhibit good adhesion in the absence of thickening or wetting agents, low resistance and good chemical and electrochemical stability.